The present invention relates to an eye-safe optical fibre transmitter unit, and to a method of producing such an unit.
Optical devices such as laser transceiver units often have an optical port for receiving and/or transmitting laser light from/into optical fibres, for example as part of an optical communications system. The optical fibre has at its end a connector by which the fibre may be connected and disconnected to the port.
In the case of an optical fibre laser transmitter unit, when the connector is not connected to the port, laser radiation from the port needs to be eye-safe. In many cases, optical communications links operate at near-infrared wavelengths of 1.3 xcexcm and 1.5 xcexcm, which present added risk because such wavelengths are invisible. Applicable eye-safety standards for infra-red laser diode transmitter units are the U.S. Standard CDRH Class 1 and the European Standard IEC 825.
Current laser safety guidelines require that the output power density from an optical port of an optical transmitter unit be limited to a level which is eye-safe when no fibre optic connected is connected to the port. Optical coupling efficiencies from a laser diode into an optical fibre are typically quite low, for example of the order of about 1% to 25%. Even if the amount of laser radiation transmitted by the fibre is eye-safe, the total amount of optical radiation emitted by the laser diode may far exceed the limit of eye-safety. It is therefore necessary either to block unwanted light within the port, or to defocus stray light emitted by the port when no optical connected is connected to the port.
One solution to this problem is disclosed in patent document U.S. Pat. No. 5,315,680, which describes an optical port having a short length of optical fibre, called a xe2x80x9cfibre stubxe2x80x9d held securely in alignment with a laser diode concealed within an optical transmitter unit. Collimating optics are used to focus the laser light into a single-mode core of the fibre. The fibre stub is typically 5 mm to 6 mm long. Light which is not coupled into the core entered the fibre optic cladding, and is dissipated by multiple reflections and scattering with the core and which the exterior surface of the cladding. Any laser radiation that exists from the cladding in not collimated, and is essentially xe2x80x9cdefocussedxe2x80x9d so that the inherent brightness of such stray radiation is greatly reduced.
In recent years there has been an increasing demand for fibre optic communication links having a bandwidth in excess of 1 GHz, for example up to 10 GHz. One way in which a laser diode can be made to operate at higher data rates is to drive the laser at a higher power. It may be possible to reduce the amount of optical power launched into the core by defocussing a laser beam focussed on the input end of the fibre stub, that is, by axially offsetting the laser beam waist with respect to the entrance face of the fibre optic core. Such a technique may also be used to vary (i.e. reduce) the amount of optical power in the core depending on product specifications and the requirements of various applications. The core diameter is, however, much smaller than that of the cladding, and so more defocussed light will be launched into the cladding. Thus, there will still be more total laser power launched into both the core and the cladding of the fibre stub, to the point where light emitted by the fibre core and/or stub is no longer eye-safe.
Another problem with using the defocus technique is that the amount of laser power entering the core then becomes more sensitive to changes in the relative orientation along the light transmission direction, of the fibre stub, the laser and any intervening collimating optics. Such orientations can change owing to thermal expansion of components forming the optical transmitter unit, or because of ageing-induced creep of the materials and adhesives used in the construction of the unit.
One way to reduce the laser power emitted at the end of the fibre stub is to increase the length of the stub in order to increase scattering and absorption over the length of the stub. Cladding modes within a length of optical fibre between about 100 mm and 200 mm long will be substantially dissipated. This, however, results in an increase in the size of the optical transmitter module, which is undesirable.
Another solution is to incorporate an aperture at the end of the stub, for example by means of an absorbing ring around the outside of the fibre core. The aperture, however, must be formed in close alignment with the core, which is of the order of about 10 xcexcm in diameter. This results in additional process steps, which add cost and complexity to the optical transmitter unit.
It is an object of the present invention to provide a more convenient eye-safe optical transmitter unit, and a method for manufacturing such a unit.
Accordingly, one aspect of the present invention provides an optical transmitter unit comprising a laser diode for emitting optical radiation. A optical fibre stub has a fibre core for carrying said optical radiation and surrounded by a cladding. The optical fibre stub is disposed in a ferrule. Focussing optics focusses said optical radiation from the laser diode into an entrance face of the fibre stub. The focussing optics focusses the optical radiation along a focus axis to a focus spot on the entrance face of the fibre stub, wherein the entrance face is at a particular orientation with respect to said focus axis The ferrule is rotatable in a manner which alters the particular orientation of the entrance face with respect to said focus axis, thereby affecting the efficiency of coupling said optical radiation into the fibre core and cladding.
Another aspect of the invention also provides an optical transmitter unit comprising a laser diode for emitting optical radiation. An optical port is connected to an optical fibre transmission link. An optical fibre stub with a fibre core carries the optical radiation to the optical port. The core is an index-guided core surrounded by cladding. Focussing optics focusses said optical radiation from the laser diode into an entrance face of the fibre stub. The focussing optics focusses the optical radiation along a focus axis to a focus spot on the entrance face of the fibre stub to increase the coupling efficiency of optical radiation into the fibre core and to decrease the coupling efficiency of optical radiation into the surrounding cladding. The coupling efficiency into the core is a maximum at a particular orientation of the entrance face with respect to the focus axis when the focus spot is on the entrance face. The entrance face is not oriented at the particular orientation, but is angled and/or rotated away from the particular orientation in order to reduce the coupling efficiency of the optical radiation into the fibre core and cladding.
Also according to another aspect of the invention there is provided a method of assembling an optical transmitter unit including a laser diode for emitting optical radiation, an optical port for connection to an optical fibre transmission link, an optical fibre stub with a fibre core for carrying the optical radiation to the optical port. The core is an index-guided core surrounded by cladding. Focussing optics focusses the optical radiation from the laser diode into an entrance face of the fibre stub. The method comprises the steps of:
i) using the focussing optics to focus the optical radiation along a focus axis to a focus spot on the entrance face of the fibre stub in order to increase the coupling efficiency of optical radiation into the fibre core and to decrease the coupling efficiency of optical radiation into the surrounding
ii) orienting the entrance face with respect to the focus axis to a particular orientation in order to maximise the coupling efficiency into the fibre core when the focus spot is on the entrance face; and then
iii) changing the orientation of the entrance face with respect to the focus axis away from the particular orientation by angling and/or rotating the entrance face with respect to the focus axis in order to reduce the coupling efficiency of the optical radiation into the fibre core and cladding.
By reducing the coupling efficiency of coupling optical radiation into the fibre stub, the amount of optical radiation emitted by the transceiver unit when no optical transmission link is connected to the transmitter unit can be reduced, and so made eye-safe. At the same time, by focussing the optical radiation onto the entrance face of the fibre stub, the proportion of optical radiation in launched into the index-guided core with respect to the cladding, is increased, so increasing the useful proportion of optical radiation transmitted by the optical transmitter unit, compared with wasted optical radiation in the cladding.
One way in which the relative orientation of the entrance face and the focus axis can be set in order to reduce the coupling efficiency is to adjust and set the angle of the entrance face with respect to the focus axis at a non-normal angle. Unwanted optical radiation will then be reflected by the entrance face, rather than admitted and transmitted within the fibre stub.
Apart from the optical port, the optical transceiver unit is preferably light-tight so that such stray reflections within the unit are ultimately absorbed.
The entrance face of the fibre stub may include a polarising element, for example a polarising isolator for suppressing back-reflection of the optical radiation into the laser diode. Such an isolator may be adhered onto an input facet of the fibre stub within the optical transceiver.
The polarising element, whether or not the element is also an isolator, may also be set at an angle or orientation at which said polarising element reduces the coupling efficiency. This can be done by rotating the polariser about the focus axis to an orientation at which the polariser admits a reduced amount of optical radiation into the fibre stub. The polariser then absorbs an increased amount of optical radiation prior to transmission of said optical radiation to the optical port by the fibre stub.
In a preferred embodiment of the invention, the focussing optics have an optical axis that is offset to the projection axis of the optical radiation from the laser diode, and the optical fibre stub has a transmission axis that is angled to a focus axis for optical radiation focussed onto the entrance face of the fibre stub. The entrance face is therefore oblique, that is, angled at a non-right angle, to the transmission axis of the optical fibre stub in order to maximise coupled optical radiation into the optical fibre stub at an optimal orientation between the entrance face of the optical fibre stub and said focussed optical radiation. The optical radiation coupled into the fibre core is then reduced by rotating the optical fibre about the transmission axis whilst otherwise maintaining the orientation between the optical fibre stub and the focussing optics.
By changing the angle and/or rotation as described above, it is possible to vary the coupling efficiency over a range whereby the amount of optical radiation coupled into the fibre core is reduced by at least 10%, and up to 90%, from the maximum coupling efficiency at said particular orientation.
The invention will now be described in further detail, and by way of example only with reference to the drawing.